The present invention relates to methods and apparatus for controlling the torque/speed characteristics of a polyphase motor and/or generator.
Polyphase machines, such as permanent magnet machines, synchronous machines, and wound rotor machines must be driven such that the windings thereof are energized as a function of the rotor position (and, thus, the rotor flux) in order to obtain driving torque from the machine (for motoring) and/or to obtain opposing torque to from the machine (for generating).
Polyphase machines may be utilized as variable starter-generator machines for an engine, such as a turbine engine. A turbine engine is started by using the polyphase machine to apply torque to a main shaft of the turbine engine, while also providing fuel and other combustion elements to the engine. When the polyphase machine is controlled to produce a suitable torque verses speed characteristic, the turbine engine will start. At or some time after the start event, the polyphase machine may stop applying torque to rotate the turbine engine and the polyphase machine may be controlled in such a way as to generate electricity in response to torque applied to the polyphase machine by the turbine engine.
A desirable torque verses speed characteristic of the polyphase machine acting as a motor to start a turbine engine includes a particular peak torque (or range of torques) substantially at the ignition speed of the turbine engine. When the torque produced by the polyphase machine is too high at the ignition speed, then any number of mechanical linkages of the turbine engine may be overstressed. Conversely, when the peak torque produced by the polyphase machine is too low at the ignition speed, it may take an excessive period of time to reach the start event. An excessive torque or an insufficient torque condition at the ignition speed leads to undesirable results. For example, overstressing mechanical linkages within the turbine engine reduces engine life and decreases the mean time between failures (MTBF). Similarly, excessively long engine start conditions result in increased engine temperature (as it is typical that no air venting in the engine exists during startup), wasted fuel, reduced engine life, decreased MTBF, and false starts.
The conventional approach to designing the polyphase machine and a control system therefore is to optimize the design of the polyphase machine as a generator. This is so because, for example, in aeronautics the polyphase machine is utilized as a generator on the order of 99% of the time and is used as a motor 1% of the time or less. Unfortunately, optimizing the characteristics of the polyphase machine as a generator does not result in an optimum design of the polyphase machine as a motor. The conventional design approach also dictates that the control of the polyphase machine as a motor establishes a fixed lead angle of the electromagnetic field of a stator of the polyphase machine as compared with the electromagnetic field of a rotor of the polyphase machine. Thus, the electrical characteristics of the polyphase machine acting as a motor are carried over and accepted as a necessary result of optimizing the polyphase machine as a generator. However, this leads to undesirable results in connection with controlling the polyphase machine as a motor. For example, a controller and driver of the polyphase machine may need to be relatively oversized as compared with a controller and driver designed for an optimized motor (instead of a generator). Alternatively, the proper peak torque at ignition characteristic might not be achievable in a polyphase machine that has been optimized as a generator. In this situation, the characteristics of the polyphase machine as a generator may need to be sacrificed for achieving a desirable torque verses speed motoring characteristic. Unfortunately, this may cause undesirable results during a generating mode (which is the more likely mode of operation), and increased machine weight.
The conventional control techniques of polyphase machines have also been unsatisfactory in connection with starting the polyphase machine at zero speed. Indeed, many kick start techniques involve complex closed loop circuit configurations that are costly.
Another shortcoming of the conventional control techniques of wound rotor polyphase machines involves the excitation voltage for the rotor. Conventional techniques call for relatively high peak-to-peak AC characteristics with corresponding high peak currents, to drive the rotor winding. This creates relatively high power losses and electromagnetic interference profiles.
Conventional control techniques of polyphase machines have also been unsatisfactory in connection with so-called soft start techniques and so-called soft stop techniques inasmuch as the control technologies have been somewhat complex and costly. Without soft start and soft stop control technologies, the polyphase machine may introduce sharp mechanical impulses during ignition, which reduces the useful life of engine bearings.
The conventional polyphase machine control techniques have also been deficient in the area of battery usage during motoring. In particular, the conventional control techniques call for the same torque/speed control profile for the polyphase machine during startup no matter how many startup attempts are made. Unfortunately, a single startup attempt may utilize 50% or more of the battery capacity in an aircraft. Thus, multiple start attempts could easily deplete the battery. A related problem is that a lower battery voltage requires an increase in the current drawn from the battery to achieve the same torque/speed profile, thereby invoking undesirable circumstances resulting from increased current draw from the battery.
Accordingly, there are needs in the art of new methods and apparatus for controlling a polyphase motor that produce more optimum torque versus speed characteristics from the polyphase machine during motoring and/or generating.